Editorial: Silicon cycling in agricultural soils under current anthropogenic influences

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Mobilité physique et chimique du 10Be, 137Cs et 210Pb (xs) dans les sols : rôle des caractéristiques pédologiques.

Les processus de transferts de matières verticaux dans les sols (bioturbation et lessivage) sont jusqu’à présent mal contraints bien qu’ils soient responsables de l’évolution temporelle des services écosystémiques rendus par les sols. Afin de mieux contraindre ces processus, une méthode de quantification cinétique basée sur les profils verticaux de 210Pb (xs), 10Be et 137Cs a été développée pour les Luvisols, dans lesquels les transferts solubles peuvent être négligés. Les limites d’applicabilité de ce type d’approche n’ont cependant pas été testées. Dans ce travail, nous avons analysé le 210Pb (xs), 10Be et 137Cs dans des sols présentant des gradients de carbone organique, de pH et de teneurs en < 2 μm (Podzol, Andosol, Ferralsol, Leptosol). Des sols acides et des teneurs en matières organiques importantes ont été considérés. La distribution verticale des isotopes étudiés a été interprétée en fonction de la teneur en matière organique, du pH et de la fraction < 2 μm. Nous démontrons que des pertes de 10Be et de 137Cs sous forme solubles sont présentes dans les Podzols, spécialement sous forêt, plus acide, pour le dernier. Des transferts solubles de 10Be ont également été identifiés dans l’Andosol mais considérés comme négligeables dans le Leptosol. Pour le 210Pb (xs), les transferts solubles sont également probables pour les sols au pH inférieurs à 5.5 (Andosol et Ferralsol) et une forte affinité pour la matière organique a été démontrée. Ainsi, en conditions acides et teneur en particules < 2 μm faible, les transferts solubles ne peuvent plus être négligés

Soil-plant silicon dynamics in natural ecosystems and agroecosystems

Silicon (Si) is widely recognized as an important regulator of the global carbon (C) cycle via its effect on diatom productivity in oceans, and as a beneficial plant nutrient, improving resistance to herbivory and pathogens and mitigating the negative effects of several abiotic stresses. This thesis explores the long-term dynamics of Si in terrestrial ecosystems, and investigates some factors driving soil-plant Si dynamics in agroecosystems. The main study sites are three 2-million-years dune chronosequences located on a climatic gradient in southwestern Australia. Within a chronosequence, plant productivity is limited by nitrogen (N), then by phosphorus (P), as soils age. We show that soil Si dynamics is primarily driven by geochemical processes in young and middle-aged soils (carbonates dissolution, clay formation, quartz enrichment), but increasingly by biological processes (silica formation in plants followed by its dissolution in soils) in old and highly-weathered soils. A climate-driven increase in biomass production along the climatic gradient seems to enhance this biological Si feedback loop. Besides, the continuous increase in community-level leaf Si concentrations with increasing soil age and P depletion might reflect the importance of silica-based defenses in P-poor environments. This increase is associated with a decrease in leaf total phenol concentrations, suggesting a tradeoff between both defense strategies along N-P gradients. We also propose that the increase in nutrient-acquisition carboxylate-releasing strategies with increasing soil age might explain the increase in leaf Si concentrations, with carboxylates not only mobilizing rhizosphere P, but also Si. Based on the above results and a literature review, we then summarized the biotic and abiotic controls on soil Si dynamics, and wondered whether they could be exploited in agroecosystems. We particularly stress the importance of mycorrhizal associations, silicate-solubilizing bacteria, soil macrofauna, root exudates and large herbivores on soil-plant Si dynamics. These ecological processes might in turn be exploited in cereal-legume intercropping, cover crops implementation, or integrated crop-livestock systems. We finally demonstrate that soil properties and recycling crop residues strongly influence the foliar silicification and its beneficial effects for two crop species (rice and sugarcane) through two case studies in Burkina Faso and Guadeloupe, respectively. This thesis highlights the major influence of soil age and weathering degree on soil-plant Si dynamics, from both a biogeochemical and ecological perspective, and demonstrate that knowledge from complex natural systems might help to improve the Si-use efficiency and subsequent sustainability of modern agroecosystems. Besides, this thesis stresses the need to develop multidisciplinary approaches to better understand elements mobility in natural ecosystems and agroecosystems

Permaculture practices effect on soil fertility and organic matter distribution in aggregate-size classes

The agricultural transition must address the need for a sustainable agricultural productivity. In this regard, the role played by soil organic matter (SOM) is key. Here we aimed to study the impact of permaculture and biointensive micro-gardening practices, mainly characterized by (1) intensive cultivation (2) the use of large and localized organic inputs and (3) the non-use of mineral fertilizers and pesticides, on soil fertility parameters and SOM distribution in aggregate-size fractions. In identical geopedoclimatic conditions, we compared SOM allocation in a pasture that evolved towards cultivation under permaculture practices for seven years and with a soil under conventional agriculture practices. For this purpose, soils were separated into three aggregate-size fractions (250-2000, 50-250 and <50 µm) by wet sieving. Then, the 250-2000 µm fraction (macroaggregates + free coarse particulate organic matter (POM)) were separated into coarse POM (free + occluded), occluded 50-250 µm and occluded <50 µm fractions. Organic carbon (OC) concentrations were measured in the 250-2000 µm, 250-50 µm, <50 µm and occluded 50-250 µm fractions and soil fertility parameters on the bulk soils. Our dataset shows that permaculture practices increased the concentrations of bioavailable nutrients Ca, Mg, K and P extracted with EDTA-ammonium, total nitrogen concentrations and OC stocks in bulk soils, which was explained by the very large manure inputs characteristics of this practice. Permaculture practices had only little effect on soil aggregation, except an increase of coarse POM proportion at the expense of the 50-250 µm fraction. Despite higher OC concentrations in the occluded 50-250 µm fraction, its contribution to the increase of OC stocks was not significant. Indeed, the increase of OC stocks was mainly attributable to coarse POM. We hypothesized that time since cultivation is not sufficient to allow the complete decomposition of coarse POM in fine inter-POM, more associated with mineral particles. We can argue that permaculture/biointensive micro-gardening practices could enhance soil fertility parameters and SOM storage. However, further temporal research is needed to study the dynamic of the macroaggregates and their ability to form protected microaggregates through the important coarse POM content. In addition, a C balance should be performed to determine if the additional OC storage correspond to a net sink of CO2

Mobility of 137Cs, 10Be and 210Pb in soil as a function of the soil texture, organic matter content and pH

Vertical matter transfer processes (bioturbation and <2μm translocation) in soils and their dynamics are up to now poorly constrained although they are responsible for the temporal evolution of the ecosystem services. In order to better constrain and quantify these processes, a kinetic quantification of the involved transfers based on vertical profiles of 210Pb (xs), 10Be and 137Cs has been developed for Luvisols in which soluble transfers could be neglected. The range of applicability of this type of approach was nevertheless not tested. In this work, we analyzed 210Pb (xs), 10Be and 137Cs in soils exhibiting gradients of organic matter, pH and < 2 μm fraction (Podzol, Andosol Ferralsol, Leptosol). Acidic soils and large organic matter contents were considered. The depth distribution of the studied isotopes was then interpreted as a function of the organic matter content, the soil pH and its < 2 μm fraction. This demonstrates that 10Be and 137Cs losses occur under soluble form in the Podzol, especially under forest, more acid, for the latter. Soluble 10Be transfers were also identified in the Andosol when they were considered as negligible in the Leptosol. For 210Pb (xs), soluble transfers are also probable for soils with pH less than 5.5 (Andosol and Ferralsol) and strong affinity for organic matter has been demonstrated. Thus, under acidic conditions and low < 2 μm fraction, soluble transfer cannot neglected anymore

Permaculture practices effect on soil fertility and organic matter distribution in aggregate-size classes

The agricultural transition must address the need for a sustainable agricultural productivity. In this regard, the roleplayed by soil organic matter (SOM) dynamics in aggregates is key. Here we aimed to study the impact of permacultureand biointensive micro-gardening practices on soil fertility parameters and SOM distribution in aggregatefractions. In identical geopedoclimatic conditions, we compared SOM dynamics in a pasture that evolved towardscultivation under permaculture practices for twelve years and with a soil under conventional agriculture practices.For this purpose, soils were separated into three aggregate-size fractions (2000-500, 250-50 and <50 µm) by wetsieving. Macroaggregates (2000-250 µm) were separated into coarse particulate organic matter (POM), microaggregateswithin macroaggregates (250-50µm) and silt and clay (<50µm). Organic carbon (OC) concentrationswere measured in each fraction and soil fertility parameters on the bulk soils. Our dataset shows that permaculturepractices increased the concentrations of bioavailable nutrients Ca, Mg, K and P extracted with EDTA-ammoniumand OC stocks in bulk soils, which was explained by the very large organic inputs characteristics of this practice.The percentage of stable macroaggregates in the permaculture plots was 5 times higher than in the conventionalsystem and 1.5 times higher than in the reference soil under pasture. Furthermore, OC concentrations in macroaggregatesand in microaggregates within macroaggregates increased after cultivation with permaculture practicesbut remained identical for the free microaggregates and silt & clay pools, compared to the pasture reference plot.Despite higher OC concentrations in the protected microaggregates fraction, its contribution to the increase ofOC stocks was significant only for one of the three permaculture plots compared to the control pasture soil. Theincrease of OC stocks was mainly attributable to coarse POM contained in the macroaggregates. As C inputs tosoil were considerably increased under permaculture practices, we hypothesized that (1) mineral associated fractionsbecome C-saturated and that additional inputs result only in an increase of labile soil C fractions such ascoarse POM occluded in the macroaggregates or (2) time since cultivation is not sufficient to allow the completedecomposition of coarse POM in fine inter-POM, more associated with mineral particles. We can argue that permaculture/biointensivemicro-gardening practices could enhance soil fertility parameters and SOM storage. However,the results are predominantly driven by important organic inputs in the permaculture soils and we showed thata large proportion is probably rapidly mineralized. Hence, further research is needed to optimize the amount oforganic inputs required to provide high fertility parameters and SOM storage under persistent forms. Furthermore, as OM inputs to soil come from other plots of the farm, a C balance should be performed at this scale, in order to determine if OC storage in cultivated plots corresponds to a net sink of CO2 at the farm scale

Permaculture practices effect on soil fertility and organic matter distribution in aggregate size classes

The agricultural transition must address the need for a sustainable agricultural productivity. In this regard, the roleplayed by soil organic matter (SOM) dynamics in aggregates is key. Here we aimed to study the impact of permacultureand biointensive micro-gardening practices on soil fertility parameters and SOM distribution in aggregatefractions. In identical geopedoclimatic conditions, we compared SOM dynamics in a pasture that evolved towardscultivation under permaculture practices for twelve years and with a soil under conventional agriculture practices.For this purpose, soils were separated into three aggregate-size fractions (2000-500, 250-50 and <50 µm) by wetsieving. Macroaggregates (2000-250 µm) were separated into coarse particulate organic matter (POM), microaggregateswithin macroaggregates (250-50µm) and silt and clay (<50µm). Organic carbon (OC) concentrationswere measured in each fraction and soil fertility parameters on the bulk soils. Our dataset shows that permaculturepractices increased the concentrations of bioavailable nutrients Ca, Mg, K and P extracted with EDTA-ammoniumand OC stocks in bulk soils, which was explained by the very large organic inputs characteristics of this practice.The percentage of stable macroaggregates in the permaculture plots was 5 times higher than in the conventionalsystem and 1.5 times higher than in the reference soil under pasture. Furthermore, OC concentrations in macroaggregatesand in microaggregates within macroaggregates increased after cultivation with permaculture practicesbut remained identical for the free microaggregates and silt & clay pools, compared to the pasture reference plot.Despite higher OC concentrations in the protected microaggregates fraction, its contribution to the increase ofOC stocks was significant only for one of the three permaculture plots compared to the control pasture soil. Theincrease of OC stocks was mainly attributable to coarse POM contained in the macroaggregates. As C inputs tosoil were considerably increased under permaculture practices, we hypothesized that (1) mineral associated fractionsbecome C-saturated and that additional inputs result only in an increase of labile soil C fractions such ascoarse POM occluded in the macroaggregates or (2) time since cultivation is not sufficient to allow the completedecomposition of coarse POM in fine inter-POM, more associated with mineral particles. We can argue that permaculture/biointensivemicro-gardening practices could enhance soil fertility parameters and SOM storage. However,the results are predominantly driven by important organic inputs in the permaculture soils and we showed thata large proportion is probably rapidly mineralized. Hence, further research is needed to optimize the amount oforganic inputs required to provide high fertility parameters and SOM storage under persistent forms. Furthermore, as OM inputs to soil come from other plots of the farm, a C balance should be performed at this scale, in order to determine if OC storage in cultivated plots corresponds to a net sink of CO2 at the farm scale

Changing sources of bioavailable silicon during pedogenesis

The importance of silicon (Si) in the global biogeochemical cycles is of the utmost importance. Hence, it acts as an essential nutrient to a number of marine organisms, accounting for up to 50% of oceanic carbon fixation. Furthermore, the chemical weathering of Si-bearing minerals controls long-term sink for atmospheric CO2. In the last twenty years, several studies highlighted the key role of the biological pumping in the global Si cycle. Indeed, plant deposits amorphous silica called “phytoliths” inside the cells of stems and leaves. Once returning to soil via litterfall, phytoliths can be used by paleobotanists to reconstruct past vegetation given specific shapes for different species as well as long resilience in soils. On the other hand, some biogeochemical studies highlighted the fact that biogenic silica can be rapidly dissolved and then recycled by vegetation through Si uptake. In this PhD project, we want to study the sources of bioavailable Si for plants over soil weathering degree. More precisely, we want to test the hypothesis that phytoliths could become the main source of bioavailable Si in highly weathered soils. To do so, we sampled 7 sandy soils over a long-term chronosequence in SW Australia including Holocene (≤ 6.5 ka), Middle Pleistocene (120-500 ka) and early Pleistocene (~ 2000 ka) dunes. Pedogenesis includes decarbonation, iron oxides release, desilification and strong eluviation. For each chronosequence stage, we sampled by pedogenic horizon, to a minimal depth of 1.5 m. We measured the so-called “bioavailable Si” in each horizon, using a 0.01M CaCl2 extractant. We also performed some XRD analysis. Si extracted with CaCl2 strongly increased with the end of decarbonation, from the Holocene to Middle Pleistocene dunes (from 2 to 7.5 mg.kg-1). It then slightly decreases until the most weathered soil, developing on an Early Pleistocene dune (around 3 mg.kg-1). We noticed that the highest concentrations of bioavailable Si have been encountered right after the decarbonation, during the iron release process and the formation of secondary minerals such as kaolinite, detected only in those soils. Henceforth, we suggest that secondary Si-bearing minerals have probably the strongest potential for releasing Si into soil solution. However, the bioavailable Si concentration in the highly weathered soil (E horizon of at least 5 meters) remains higher than in the first stages of the sequence, with an XRD analysis showing 100% of quartz. In this system, it is likely that phytoliths have become the only source releasing bioavailable Si into soil solution. However, it is premature to draw general conclusions and phytoliths physical extraction, total elemental analysis and the use of other extractants (acetic acid, Na2CO3 following the ratios Si/Al, Si/Fe and Si/Mg) will be very useful to go much further in the hypothesis

Permaculture practices effect on soil fertility and organic matter distribution in aggregate-size classes

The agricultural transition must address the need for a sustainable agricultural productivity. In this regard, the role played by soil organic matter (SOM) dynamics in aggregates is key. Here we aimed to study the impact of permaculture and biointensive micro-gardening practices on soil fertility parameters and SOM distribution in aggregate fractions. In identical geopedoclimatic conditions, we compared SOM dynamics in a pasture that evolved towards cultivation under permaculture practices for twelve years and with a soil under conventional agriculture practices. For this purpose, soils were separated into three aggregate-size fractions (2000-500, 250-50 and <50 µm) by wet sieving. Macroaggregates (2000-250 µm) were separated into coarse particulate organic matter (POM), microaggregates within macroaggregates (250-50µm) and silt and clay (<50µm). Organic carbon (OC) concentrations were measured in each fraction and soil fertility parameters on the bulk soils. Our dataset shows that permaculture practices increased the concentrations of bioavailable nutrients Ca, Mg, K and P extracted with EDTA-ammonium and OC stocks in bulk soils, which was explained by the very large organic inputs characteristics of this practice. The percentage of stable macroaggregates in the permaculture plots was 5 times higher than in the conventional system and 1.5 times higher than in the reference soil under pasture. Furthermore, OC concentrations in macroaggregates and in microaggregates within macroaggregates increased after cultivation with permaculture practices but remained identical for the free microaggregates and silt & clay pools, compared to the pasture reference plot. Despite higher OC concentrations in the protected microaggregates fraction, its contribution to the increase of OC stocks was significant only for one of the three permaculture plots compared to the control pasture soil. The increase of OC stocks was mainly attributable to coarse POM contained in the macroaggregates. As C inputs to soil were considerably increased under permaculture practices, we hypothesized that (1) mineral associated fractions become C-saturated and that additional inputs result only in an increase of labile soil C fractions such as coarse POM occluded in the macroaggregates or (2) time since cultivation is not sufficient to allow the complete decomposition of coarse POM in fine inter-POM, more associated with mineral particles. We can argue that permaculture/biointensive micro-gardening practices could enhance soil fertility parameters and SOM storage. However, the results are predominantly driven by important organic inputs in the permaculture soils and we showed that a large proportion is probably rapidly mineralized. Hence, further research is needed to optimize the amount of organic inputs required to provide high fertility parameters and SOM storage under persistent forms. Furthermore, as OM inputs to soil come from other plots of the farm, a C balance should be performed at this scale, in order to determine if OC storage in cultivated plots corresponds to a net sink of CO2 at the farm scale